The present invention relates to a semiconductor design technology, and more particularly, to technology for supplying a programming current for programming a memory cell.
Phase change random access memory (PCRAM) is a representative nonvolatile memory device that programs memory cells by using a programming current.
PCRAM having a nonvolatile memory characteristic can provide a random access, and can be highly integrated at low cost. PCRAM stores data by using a phase change material. To be specific, PCRAM is a nonvolatile memory device using a phase change of a phase change material depending on a temperature condition, that is, a resistance change depending on a phase change.
A phase change material is changeable to an amorphous state or a crystalline state according to a temperature condition. A representative phase change material is a chalcogenide alloy. Since a typical example of the phase change material is Ge2Sb2Te5 containing germanium (Ge), antimony (Sb), and tellurium (Te), the phase change material is called a “GST”.
PCRAM causes a reversible phase change between a crystalline state and an amorphous state of the phase change material by using Joule heat generated by applying a specific current or voltage to the phase change material. The crystalline state is called a set state in terms of circuit. In the set state, the phase change material has an electrical characteristic like a metal with a low resistance. The amorphous state is called a reset state in terms of circuit. In the reset state, the phase change material has a resistance higher than in the set state. That is, PCRAM stores data on the basis of a resistance change between the crystalline state and the amorphous state, and determines the stored data by detecting a current flowing through the phase change material or a voltage change depending on a current change. In general, the set state is defined as having a logic level “0” and the reset state is defined as having a logic level “1”. The phase change material maintains its state even when power is interrupted.
Meanwhile, a programming current can make the phase change material change from the crystalline state to the amorphous state, and vice versa. A set current is defined as a programming current that changes the phase change material of the memory cell to the set state, and a reset current is defined as a programming current that changes the phase change material of the memory cell to the reset state. The set current and the reset current are also called a set programming current and a reset programming current, respectively.
If the reset current is supplied to heat the phase change material for a certain time at a temperature higher than a melting temperature, the phase change material cools rapidly and changes to the amorphous state. Also, if the set current is supplied to heat the phase change material for a certain time at a temperature higher than a crystallization temperature and lower than a melting temperature, the phase change material cools slowly and changes to the crystalline state. Meanwhile, since a resistance value is differentiated according to the crystalline volume or the amorphous volume of the phase change material, a multi-level memory cell can be implemented. In general, the reset current is made to flow with a large current for a relatively shorter time than the set current, and the set current is made to flow with a small current for a relatively longer time than the reset current. That is, the state of the phase change material is changed by Joule heating generated under a specific condition by the supply of the programming current.
FIG. 1 is a circuit diagram of a conventional phase change memory device.
Referring to FIG. 1, the conventional nonvolatile memory device includes a plurality of programming current driving units 121, 122, 123 and 124 configured to supply a programming current IPGM corresponding to a write data to memory cells, and a plurality of programming current controlling units 111, 112, 113 and 114 configured to generate a plurality of control voltages WDB1, WDB2, WDB3 and WDB4 for controlling the programming current IPGM.
When the memory cells are phase change memory cells, the magnitude and supply time of the programming current IPGM outputted from the programming current controlling units 111, 112, 113 and 114 are determined according to the pulse waveforms of the control voltages WDB1, WDB2, WDB3 and WDB4. Therefore, the memory cells are programmed to a set state or a reset state according to the supplied programming current IPGM.
As such, the control voltages are generated from the programming current controlling units provided at the respective programming current driving units. Therefore, an entire circuit area increases due to the programming current controlling units, and a lot of current is consumed due to a large number of the programming current controlling units.